REBUTTAL – Dwight has given good information below, yet I believe he left it in an Apple to Orange comparison for some info rather than a true Apple to Apple
Counter Points: Five Reasons You shouldn't switch to CNG
(1) CNG is much less dense than Gasoline. At atmospheric pressure a given volume of natural gas has 1/1000th the density of gasoline. Even at 3,000 psi (204x the pressure of the air we breathe, CNG has about 1/5th (22%) the volumetric energy density of gasoline. What it means is that for a given size of tank, a CNG car will go 1/5th the distance between refueling. Or, if it is to go the same distance it has to have 5 times the fuel tank volume.
Rebuttal – True as it comes out of the ground, but then oil is not usable as a fuel either till refined. As such, Natural Gas is compressed (CNG) to 3600 psi that gives you a Gas Gallon Equivalent (GGE). This is how you then have a usable fuel for auto. CNG GGE = 1 Gallon Petrol. MPG = Same as Petrol, 130 Octane means you get more HP and Torque depending on the efficiency of the engine. Your example of only 1/5th the distance on CNG is misleading, proven by the industry over and over.
(2) CNG is compressed to a high pressure. This means that tanks have to be cylindrical or spherical to effectively contain that pressure. Spheres and cylinders are much less space efficient than the kind of irregular shaped gasoline tanks cars use to tuck the fuel under the rear seats above and around the drive shaft and suspension bits.
Rebuttal – Yes the cylindrical shape is not as efficient in one large tank as a petrol tank, but thanks to carbon fiber, type 5 tanks allow multiple layouts by using multiple small tanks such as the Chevy Van that uses a 3 tank layout in the frame under the body to give you a impressive storage amount.
(3) CNG is not as available as gasoline or even diesel at today's gas stations. If you drive CNG, you have to plan your refueling stops around stations that sell CNG. If you drive gasoline you just drive and pull over at any gas station when the empty light goes on.
Rebuttal – All current OEM built autos have CNG fast fill stations clearly listed for ease of find a fueling station. In fact one can drive from Vancouver BC to Baja California, From LA to Florida and up and down the east coast rather easy with finding fast fill CNG stations. Even driving from San Francisco to DC across the middle of the US was done on a CNG road trip in a CNG only Honda Civic GX. The 2015 Bi-Fuel Chevy Impala, with Petrol and CNG, this car on sale spring of 2014 will have a 150 mile range on CNG plus the petrol range.
On top of this, with CSA certified CNG home fueling appliances, you can fuel for less than a dollar at home not having to bother going to a CNG station. As of 2009, 50% of all households in the US had Natural Gas available for use. http://www.eia.gov/tools/faqs/faq.cfm?id=49&t=8
(4) Converting existing engines to CNG has no performance or efficiency benefits. In fact, both are a little worse. CNG is best run with increased compression ratios, but most converted CNG engines simply replaces gasoline fuel systems and metering with a CNG compatible alternative without changing the engine internals. If you drive CNG, it is best to get a factory CNG vehicle with an engine designed from the ground up to use CNG. These unfortunately are few and far in between, limiting your choices.
Rebuttal – Todays conversion kits allow the modern day efficient engines to burn CNG so that one keeps the same MPG. Fuel delivery via the injectors, a fully functional computer brain that takes all sensors into account and adjust timing / dwell, etc. to maximize the burn of CNG.
Can the engines be tuned or built to be more efficient for CNG, sure and that has been done in markets like Italy and Brazil. In many places, these pure CNG engines are just that, the current Petrol engines tweaked to truly maximize CNG. Yet you do not loose MPG with a Petrol engine converted to CNG. One exception is older auto’s that have carburetors, the adapters used for these engines do reduce MPG, HP and torque due to the inefficient use of fuel via the Carburetor. Modern day engines with injectors loose no fuel loss and as such no loss of MPG, HP or torque.
(5) Today, US Natural Gas usage in vehicles is about 33 billion cu-ft compared to the total 23,400 billion cu-ft. That is 0.14% of the total usage; quite insignificant in the overall scheme of things. The USA uses a lot of NG and for good reasons we have a lot of it. Can we use more and be less reliant of imported oil? Sure. But is NG in vehicular use the best avenue to increase that usage? In residential and industrial heating, as well as power generation, the storage density issues (CNG's Achilles heels) are largely irrelevant since the fuel is pipe delivered in very mildly pressurized form. Most US power plants are not NG fired. Many homes use electric stoves and heaters. A drive to convert these to NG has a much larger effect on NG usage than trying to use them in vehicles without all the compromises.
Rebuttal – According to the EIA, 35% of electrical production is now by Natural Gas, 65% is an almost 50/50 mix of Coal and Nuclear. Natural Gas use has finally passed coal use for clean energy production. Natural gas is far more readily available to home users and with efficient CSA certified CNG appliances, Time Fill fueling at home is a reality. While we see many businesses such as UPS and Waste Mgmt. go to CNG fueled fleets, more and more trucks are being produced to run on pure CNG only by Kenworth and Peterbuilt for inner city deliveries as the cities require cleaner fleets. Cost of CNG for businesses and home owners who keep their auto's longer than 3 yrs makes sense to use this abundant fuel.
As one can see from the map below, we have an extensive network of Natural Gas pipelines and it is continually expanding.
Here is the compressor map showing the ability to move natural gas around the US.